1. Required properties of refrigerating machine oil:
Typical type of compressor for refrigerators are a reciprocating type, a scroll type, and a rotary type. In these types, a rotary compressor is used under severe conditions, like a high-temperature and high-pressure condition.
Important properties that are generally taken into account for a refrigerating machine oil used in any types of compressors are wear resistance, load carrying capacity, thermal stability, chemical stability, low-temperature fluidity, and compatibility with a refrigerant.
The refrigerating machine oil is required to have functions of preventing wear of and cooling a sliding portion of the compressor, releasing heat generated upon compression of the refrigerant, sealing at a refrigerant-compressing step, and removing worn powders and foreign matters, etc.
Thus, the refrigerating machine oils having not only excellent lubricating properties (such as wear resistance, load carrying capacity, etc.,) but also high thermal and chemical stabilities in the compressor and giving no adverse influences on the machine parts (e.g., metals, etc.) of the compressor are desired.
Also, a part of the refrigerating machine oil is carried with a compressed refrigerant, circulates through the system of the refrigerator, and flows into low-temperature portions such as an evaporator, capillary tubes, expansion valves, etc. Thus, for increasing the cooling performance of the evaporator and improving recovery of the oil from the low-temperature portions to the compressor, or for supplying the oil to sliding portions of the compressor at a low temperature when resuming the operation, good low-temperature fluidity and good compatibility with the refrigerant are required for the refrigerating machine oil.
2. Relation of refrigerant and refrigerating machine oil:
Hitherto, as a refrigerant used in a compressor for a refrigerator, Flon series refrigerants such as a chlorofluorocarbon (CFC) series refrigerant and a hydrochlorofluorocarbon (HCFC) series refrigerant have been used independently or as a mixture thereof. These refrigerants have good compatibility with a non-polar hydrocarbon oil since they have a low polarity. Also, the Flon series refrigerants each has chlorine atoms in the molecule, so that the chlorine reacts with a material constituting the sliding surface of the compressor to form a chloride which acts as a lubricant. In addition, a hydrocarbon oil has a good lubricating property.
For the reasons, hydrocarbon oils such as properly refined naphthenic mineral oils, paraffinic mineral oils, alkyl benzenes, poly-.alpha.-olefines, etc., are used independently or as a mixture thereof as a base oil of a refrigerating machine oil for the refrigerator using the Flon series refrigerant, to which an antioxidant, an anti-wear agent, a corrosion inhibitor, etc., are generally added.
In this connection, a phosphate has a low solubility in a hydrocarbon oil and shows a wear resistance effect at a low concentration. Therefore, the phosphate is usually added to the base oil (hydrocarbon oil) in an amount of not more than 1% by weight.
After being reported that the ozone layer in the stratosphere is destroyed by Flon containing chlorine atoms, the regulation on use of the CFC series refrigerant and the HCFC series refrigerant becomes strict worldwide. Under the circumstance, intensive studies for the substitution thereof has been made, and various substitutes have been reported for the Flon series refrigerants (e.g., for HCFC-22 (R-22)), such as hydrofluorocarbon (HFC) series mixed refrigerants, e.g., HFC-134a, HFC-143a, HFC-125, HFC-32, etc.
However, since the HFC series refrigerants have a high polarity, they have poor compatibility with a hydrocarbon oil. Thus, a refrigerating machine oil suitable for the HFC series refrigerants has been desired.
3. Conventional techniques on refrigerating machine oils for HFC series refrigerants:
As a lubricating oil for a refrigerator using the HFC series refrigerant, synthetic oxygen-containing hydrocarbon oils having good compatibility with the HFC series refrigerants, such as ester series synthetic oils, polyether series synthetic oils, etc., have been known. Among these synthetic oils, the ester series synthetic oils have high electric insulating property, good compatibility at a high-temperature and low hygroscopic property, as compared with the polyether series synthetic oils.
There have been known refrigerating machine oils composed of a ester series synthetic oil, as disclosed in, for example, JP-A-56-133241 and JP-A-59-164393 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"); refrigerating machine oils to be used with a chlorinated fluorinated hydrocarbon or a fluorinated hydrocarbon for the refrigerant, as disclosed in JP-A-2-276894; and refrigerating machine oils to be used with a hydrogen-containing Flon, as disclosed in JP-A-3-88892, JP-A-3-128991, and JP-A-3-128992.
Also, refrigerating machine oils comprising an ester series synthetic oil and a phosphate or a phosphite are disclosed in JP-A-55-92799, JP-A-56-36570, JP-A-56-125494, JP-A-62-156198, JP-A-3-24197, and JP-A-5-59388, and oils for a heat pump are disclosed in JP-B-57-43593 (the term "JP-B" as used herein means an "examined Japanese patent application").
In particular, the foregoing refrigerating machine oil disclosed in JP-A-5-59388, which is also applicated in U.S. and issued as U.S. Pat. No. 5,342,533, is for a refrigerator using the HFC series refrigerant and consists essentially of 100 parts by weight of a dibasic acid diester or a carboxylate of a polyhydric alcohol as a base oil, and from 5.0 to 90.0 parts by weight of a phosphate or a phosphite.
Furthermore, refrigerating machine oils containing a thiophosphite, an epoxy compound and a methanesulfonate are disclosed in JP-A-56-36569, JP-A-58-15592, and JP-A-62-292895. Also, refrigerating machine oils comprising an ester oil, an alkylbenzene, or a mineral oil as a base oil and an alkylene glycol glycidyl ether or an aliphatic cyclic epoxy compound having a specific structure are disclosed in JP-A-5-17792.
A polyol ester (an ester series synthetic oil) exhibits excellent electric insulating property, compatibility with the HFC series refrigerant at a high-temperature, and a low hygroscopic property, and hence the polyol ester is preferable for the refrigerator using the HFC series refrigerant.
However, since the polyol ester is chemically active as compared with a hydrocarbon oil, the polyol ester is liable to form a sludge in a compressor at a high temperature. Also, since the HFC series refrigerant does not have a chlorine atom in the molecule, the lubricating property becomes insufficient sometimes in the case of using a compressor under severe conditions. Hitherto, efforts have been made to improve a wear resistance and a thermal stability by way of additives, but satisfactory additives enabling suppression of a sludge formation and a prevention of wear each being the problems encountered in use of the ester series synthetic oils, has not yet been developed.
In particular, a rotary compressor is used under severe conditions as compared with the case where a reciprocating compressor or a scroll compressor is used, and hence the refrigerating machine oil used in the rotary compressor is required to have a higher wear resistance and a higher thermal stability simultaneously. Thus, for the refrigerator equipped with a rotary compressor, it is considered to be difficult to use a polyol ester, and hence the improvement of the inside parts of the compressor has been attempted.